Method and apparatus for scanning transmission electron microscopy

ABSTRACT

A scanning transmission electron microscope (STEM) has an electron source for generating a primary electron beam and an electron illuminating lens system for converging the primary electron beam from the electron source onto a specimen for illumination. An electron deflecting system is provided for scanning the specimen with the primary electron beam. The STEM also has a scattered electron detector for detecting scattered electrons transmitted through the specimen. A projection lens system projects the scattered electrons onto a detection surface of the scattered electron detector. An image displaying device displays the scanning transmission electron microscope image of the specimen using a detection signal from the scattered electron detector. A detection angle changing device for establishes the range of the scattering angle of the scattered electrons detected by the scattered electron detector. This structure enhances the contrast of a desired portion of the specimen under observation for a scanning transmitted image by selective establishment of detection angle ranges for the scattered electron detector.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning transmission electronmicroscope and a method of scanning transmission electron microscopyapplicable to the scanning transmission electron microscope which offerssubstantially the same ease of operation as that of scanning electronmicroscopes and which provides substantially the same degree ofresolution as that of transmission electron microscopes.

A scanning transmission electron microscope (STEM) based on aconventional transmission electron microscope (TEM) is disclosedillustratively in Japanese Patent Laid-open No. 016160/1977. JapanesePatent Laid-open No. 036763/1982 discloses structures of a secondaryelectron detector for use with a TEM-based STEM. With TEM-based STEMs,the axial alignment of their electron lens system is accomplishedconventionally using a fluorescent screen in a dark room to observeelectron beam paths.

Expectations were high for the advent of scanning transmission electronmicroscopes (STEM) which have levels of resolution as high as those oftransmission electron microscopes (TEM) and which may be operated aseasily as scanning electron microscopes (SEM). The above-mentionedTEM-based STEM can be a bulky machine about 2.5 m in height to house acomplicated electron lens system for imaging diffraction patternsderived from transmitted electrons. Where to install such a tallTEM-based STEM has turned out to be a problem. With the TEM-based STEM,images have often been observed by use of a fluorescent screen, whichrequires a dark room. Furthermore, an experienced operator's skills andknowledge have been necessary for the alignment of the optical axis ofthe TEM-based STEM.

Meanwhile, the scanning transmission electron microscope (STEM) based onscanning electron microscopes (SEM) has no image-magnifying lens system.That means the alignment of an electron lens system is not available onthe SEM-based STEM. In addition, the SEM-based STEM is lacking in theease of operation in that the preparation and attachment of specimens aswell as the feeding of operating instructions to the machine cannot beaccomplished on a display screen.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a scanningtransmission electron microscope (STEM) which has a level of resolutionas high as that of transmission electron microscopes (TEM) and which maybe operated as easily as scanning electron microscopes (STEM).

It is another object of the invention to provide a scanning transmissionelectron microscope (STEM) which permits the observation of scanningtransmitted images of specimens in a well-lighted room.

It is a further object of the invention to provide a scanningtransmission electron microscope (STEM) allowing the optical axis of aprimary electron beam to be adjusted easily for observation purposes.

It is an even further object of the invention to provide an interactiveinput/output device which is used in conjunction with a scanningtransmission electron microscope (STEM). The interactive input/outputdevice facilitates the observation of scanning transmitted images ofspecimens on the microscope.

It is a still further object of the invention to provide a method ofscanning electron microscopy which is for use with a scanningtransmission electron microscope (STEM). The method allows internalstructures of specimens to be observed in a three-dimensional fashion.

In achieving the foregoing and other objects of the present invention,there is provided a scanning transmission electron microscopecomprising: an electron source for generating a primary electron beam;an electron illuminating lens system for converging the primary electronbeam from the electron source onto a specimen for illumination; anelectron deflecting system for scanning the specimen with the primaryelectron beam emitted thereto; a scattered electron detector fordetecting scattered electrons transmitted through the specimen; and animage displaying system for displaying a scanning transmission electronmicroscope image of the specimen using a detection signal from thescattered electron detector. This inventive structure provideseffectively a scanning transmission electron microscope (STEM) based ona scanning electron microscope (SEM).

In a preferred structure according to the invention, the scanningtransmission electron microscope (STEM) may further comprise a detectionangle changing system for variably establishing a range of scatteringangle of the scattered electrons detected by the scattered electrondetector. This preferred structure enhances a contrast of a desiredportion of the specimen under observation for a scanning transmittedimage, thereby improving the precision of structural and componentanalyses of the specimen.

In another preferred structure according to the invention, the scanningtransmission electron microscope (STEM) may further comprise a PC-basedinteractive input/output device with a display screen permitting inputof conditions for operating components of the scanning transmissionelectron microscope. This structure renders the microscope easier tooperate and thereby alleviates users' operating chores.

In a further preferred structure according to the invention, thescanning transmission electron microscope (STEM) may further comprise asecondary electron image displaying system for displaying a secondaryelectron image of the specimen through detection of secondary electronsreleased by the specimen, and/or reflected electron image displayingmeans for displaying a reflected electron image of the specimen throughdetection of reflected electrons from the specimen. This preferredstructure provides more aspects of observed information about thespecimen by supplementing its scanning transmitted image with asecondary electron image and/or a reflected electron image.

In an even further preferred structure according to the invention, theelectron illuminating lens system may include accelerating electrostaticlenses for accelerating the primary electron beam from the electronsource, and converging lenses for converging the accelerated primaryelectron beam onto the specimen. The electron illuminating lens systemmay further include an electron source deflector for aligning an opticalaxis of the primary electron beam from the electron source. The electronsource deflector may be constituted illustratively by a scanningdeflector and by two-stage deflectors with reversal polarity which arelocated above and below the scanning deflector.

These and other objects, features and advantages of the invention willbecome more apparent upon a reading of the following description andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a scanning transmission electronmicroscope (STEM) practiced as an embodiment of this invention;

FIG. 2 is an electron ray diagram for explaining electron opticalfunctions of the STEM in FIG. 1:

FIG. 3 is an electron ray diagram for explaining electron opticalfunctions of components ranging from a specimen to a scattered electrondetector in the STEM of FIG. 1;

FIG. 4 is a schematic view illustrating a distribution of voltagesupplies and their connections to electrostatic lenses for variableadjustment of an electron acceleration voltage in the STEM of FIG. 1;

FIG. 5 is a schematic view depicting a typical structure of an electrongun in the STEM of FIG. 1;

FIG. 6 is a schematic view showing components ranging from converginglenses to a scattered electron detector in a scanning transmissionelectron microscope (STEM) practiced as another embodiment of thisinvention;

FIG. 7 is a schematic view indicating approximate dimensions of anobjective lens included in the STEM of FIG. 6;

FIG. 8 is a graphic representation illustrating differences in angulardistribution of scattering intensity between different materials;

FIG. 9 is a schematic view showing those regions in a bulk specimenwhich emit secondary and reflected electrons;

FIG. 10 is a schematic view for explaining changes in a contrast of asecondary electron image, the changes being attributable to differentdepths in a specimen;

FIG. 11 is a flowchart of steps for observing a projected image of aspecimen structure based on a scanning transmitted image of a specimen,and for observing a three-dimensional image of the specimen using itssecondary and reflected electron images;

FIG. 12 is a flowchart of steps for observing a three-dimensional imageof a specimen using its secondary and reflected electron images;

FIG. 13 is a schematic view of a specimen and a structure for holding itwith a view to acquiring an optimal defocus amount based on scanningtransmission image intensities;

FIG. 14 is a graphic representation for explaining ways to acquire anoptimal defocus amount based on scanning transmission image intensities;and

FIG. 15 is a schematic view showing a typical setup of a specimen holderand a specimen stage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention will now be described in detailwith reference to the accompanying drawings.

Described first are ways to acquire a scanning transmitted image of aspecimen by use of a scanning transmission electron microscope (STEM)based on a scanning electron microscope (SEM) to detect scatteredelectrons having a given range of scattering angle.

FIG. 1 schematically shows a scanning transmission electron microscope(STEM) embodying the invention. An electron beam from an electron source1 is accelerated to a predetermined acceleration voltage by multi-stageelectrostatic lenses 2 a, 2 b and 2 c. The acceleration voltage perstage is set for about 30 kV, and the voltage applied to the lenselectrode at each stage is varied. This setup allows an ultimateacceleration voltage for the electron beam to be varied.

The electron beam thus accelerated up to the predetermined accelerationvoltage is converged (demagnified) by a first and a second stageconverging lens 3 a and 3 b. The demagnification of the electron beammay be altered as needed by changing the variation of exciting currentsto the first and the second stage converging lens 3 a and 3 b.

The electron beam is further demagnified by a strongly excited pre-fieldobjective lens 4. Finally, an electron probe having a subnanometer-orderdiameter is formed on a specimen 5. A convergence aperture 6 under thesecond stage converging lens 3 b is used to change the probe in itsconvergence angle, thereby balancing diffraction aberration withspherical aberration in connection with the probe diameter. Electronstransmitted through and scattered by the specimen 5 form an electrondiffraction pattern under the specimen 5. A post-field objective lens 7is strongly excited so that the diffraction pattern mentioned above isprojected under the lens with virtually no magnification. Excitingcurrents fed to a first and a second stage projection lens 8 a and 8 bunder the post-field objective lens 7 are adjusted so as to vary themagnification of the electron diffraction pattern for projection onto ascattered electron detector 9.

The scattered electron detector 9 is a detector which, incorporating aCCD, a harpicon camera or the like, provides high sensitivity, a highsignal-to-noise ratio and high linearity and which quantitativelymeasures the intensity of scanning transmitted images. The detector 9 isof an annular shape made of a disk having a concentric hole (or cover)at its center. The shape of the detector 9 is designed to exclude lowangle scattering components of electrons from a target to be detected.Alternatively, a disk-shaped detector 9 may have a cover arranged tocover the low angle scattering components bound for the detector center.A detector alignment coil 10 under the second stage projection lens 8 bis used to align the detector 9 axially with the electron diffractionpattern.

An objective aperture 11 is located on an image plane formed for anelectron diffraction pattern under the specimen 5. The aperture 11 hasits diameter changed in a plurality of steps. Alternatively, theaperture 11 may be formed by one of a plurality of (e.g, four)interchangeable holes (circular openings) with different diameters. Adesired hole may be selected from the optional holes and installed asneeded. The objective aperture 11 determines a maximum angle at which todetect an electron diffraction pattern. That is, a minimum range fordetecting scattered electrons (i.e., minimum angle) is determined by twofactors: the magnification of the scattered electron detector 9detecting an electron diffraction pattern, determined by the excitingcurrents fed to the first and the second projection lens 8 a and 8 b;and the physical inner diameter of the scattered electron detector 9. Amaximum range for detecting scattered electrons (maximum angle) isdetermined by the diameter of the objective aperture 11.

A scanning transmitted image is acquired by scanning the specimen 5two-dimensionally with the probe using a deflector coil 12 and bymodifying the brightness of the scanning transmitted image on a CRTusing a detection signal from the scattered electron detector 9 insynchronism with the scanning. More specifically, electron intensitysignals representing diffraction patterns acquired in various scannedpositions are detected as a contrast signal of a scanning transmittedimage. An image is displayed through an image reproducing processsynchronized with the above scanning (i.e., with a scanning or adeflection signal), whereby a scanning transmitted image of the specimenis acquired. Intensity levels of diffraction patterns in differentscanned positions on the specimen 5 are magnified by a pre-amplifier 42and converted to digital signals by an A/D converter 43. The digitalsignals thus converted are stored as a digital image file in a storagedevice (not shown). Contents of the image file are retrieved by a CPU 14and sent to image display equipment in an interface 15. The imagedisplay equipment displays the scanning transmitted image of thespecimen 5.

All lenses, coil components and detectors are operated by means of a D/Aconverter 13 under control of the CPU 14. Conditions for operating thecomponents are established by an operator using the interface 15. Asecondary electron detector 16 and a reflected electron detector 17 arelocated above the pre-field objective lens 4. The two detectors are usedto acquire a scanning secondary electron image and a scanning reflectedelectron image at the same time as the above-described scanningtransmitted image.

Below is a description of an electron lens system for imaging a scanningtransmitted image, a scanning secondary electron image and a scanningreflected electron image. FIG. 2 is an electron ray diagram thatgeometrically illustrates optical paths of an electron beam 30 emittedfrom a virtual source 29 for imaging onto the detector 9. The virtualsource 29 is not to be equated with a physical position of the electronsource 1; rather, the source 29 stands for an effective electron sourceposition determined by the extracting voltage and the radius ofcurvature of a tip that works as an electron source. An electron beam 30from the virtual source 29 is demagnified or magnified by the first andthe second converging lens 3 a and 3 b before entering the objectivelens. The objective lens as used herein signifies a single lens formedby a single magnetic circuit. Optically, however, the objective lensplays two parts: one to demagnify the electron beam 30, and another toform an image out of the electrons scattered by the specimen. Thepre-field objective lens 4 plays the demagnifying part, the post-fieldobjective lens 7 the imaging part.

The electron beam 30 demagnified by the pre-field objective lens 4 fallsonto and enters the specimen 5. At this point, part of the electronbeams 30 are transmitted through the specimen 5 and emitted from itsbottom, and part of the electron beams 30 are reflected inside thespecimen 5 and emitted from its top. Upon emission of the reflectedelectrons, secondary electrons are released from the surface of thespecimen 5. The secondary and the reflected electrons are detected andformed into a secondary and a reflected electron image respectively. Theelectrons transmitted through the specimen 5 are imaged by thepost-field objective lens 7 onto a probe image plane 31.

On a back focal plane 32 of the post-field objective lens 7 is formed anelectron diffraction pattern which reflects phase information about theelectrons diffracted by the specimen 5. The first stage projection lens8 a is focused on the back focal plane 32 for the electron diffractionpattern. An image plane 33 of the first stage projection lens 8 a isfocused by the second-stage projection lens 8 b onto the scatteredelectron detector 9. Varying the exciting current to the second stageprojection lens 8 b magnifies or demagnifies the image formed on thescattered electron detector 9. This in turn makes it possible for thescattered electron detector 9 to establish the range for detectingscattered electrons as desired.

FIG. 3 is an electron ray diagram showing how electrons diffracted bythe specimen 5 are arranged to form the probe image plane 31 and backfocal plane 32 by the post-field objective lens 7. Transmitted electrons40 propagating in parallel with the direction of probe incidence, anddiffracted electrons 41 propagating in different directions, are alteredas illustrated in their propagating directions by the post-fieldobjective lens 7.

On the back focal plane 32, the electrons propagating in the samedirection past the specimen 5 are converged onto a single spot. That is,on the back focal plane 32 is formed a diffraction pattern whereinelectrons are scattered depending on the angle of diffraction inside thespecimen 5. The electrons transmitted through the specimen 5 form,between the post-field objective lens 7 and the first stage projectionlens 8 a, the probe image plane 31 corresponding on a one-to-one basisto a physical surface of the specimen 5. An image formed on the probeimage plane 31 during the parallel scanning of the specimen 5 using theprobe beam moves in parallel in synchronism with the probe beamscanning, whereas the diffraction image on the back focal plane 32 doesnot move. Thus to observe a scanning transmitted image need only requirethat the first stage projection lens 8 a be focused on the back focalplane 32 to extract information dependent on the probe position, andthat the electron diffraction pattern appearing on the back focal plane32 be formed on the scattered electron detector 9.

What follows is a description of how to vary the acceleration voltagefor an electron beam generated by the electron source 1. FIG. 4schematically illustrates a typical structure of an electron gun. Theelectron gun is shown comprising an electron source 1 and electrostaticlenses 2 a, 2 b and 2 c in a three-stage arrangement, each of the lensesbeing connected to a supply voltage. The electron source 1 iselectrically grounded. The grounding is provided independent of anelectrical ground of the microscope. A first anode 18 is fed with anextracting voltage V_(EXT) (about +5 kV) so as to extract an electronbeam from the electron source 1. The first, the second and the thirdstage electrostatic lens 2 a, 2 b and 2 c are supplied respectively withacceleration voltages V₁, V₂ and V₃ relative to the ground level. Theacceleration voltage per electrostatic lens stage is approximately 30kV, which means the three-stage electrostatic lens arrangement providesa total acceleration voltage of about 100 keV. If the number of stagesfor electrostatic lenses is increased to 6 or to 10, then the finalacceleration energy for the electron beam will be about 200 keV or 300keV respectively. The range of acceleration voltages may be varied insteps of about 30 kV by selectively turning on and off accelerationvoltages to lower stage electrostatic lenses. Acceleration voltageincrements of as small as 50 mV (minimum changeable range) may beimplemented by altering a standard voltage of a voltage transformer thatgenerates the voltage applied to the last-stage electrostatic lens (V₃in the example of FIG. 4) relative to the ground level.

FIG. 5 schematically depicts how electrostatic lenses and othercomponents are illustratively arranged in an electron gun. The electrongun is shown incorporating an electron source 1, a first anode 18, andelectrostatic lenses 2 a through 2 f in a six-stage arrangement insidean accelerating tube vacuum vessel 36.

An accelerating tube evacuating vessel 38 is located under theaccelerating tube vacuum vessel 36. The evacuating vessel 38 isfurnished with a port of connection (not shown) to a turbo molecularpump, an ion pump or the like. When installed, such a vacuum evacuatingpump evacuates the inner space of the accelerating tube vacuum vessel 36to an extreme high vacuum on the order of 10⁻⁹ Pa. Outside theaccelerating tube vacuum vessel 36 is a housing 37 that contains aninsulating gas such as sulfur hexa-fluoride. Above the accelerating tubeevacuating vessel 38 is an electron gun deflector 39 that axially alignsthe electron beam inside the electron gun.

Described below is how the electron gun deflector 39 is operated toalign axially the electron beam within the electron gun. The electrongun deflector 39 comprises a scanning deflector 39 a and two-stagedeflectors with reversal polarity 39 b located above and below thescanning deflector 39 a. Each of the deflectors has an X- and a Y-axisdirection deflector coil. The electron gun deflector 39 is fed with adeflection signal for scanning the electron beam two-dimensionally at aTV-scan rate or a lower rate. In this setup, a deflection signalsupplied to the deflector coil 12 is switched over for use as adeflection signal to the scanning deflector 39 a. Since the axialalignment of the electron beam in the electron gun does not occursimultaneously with the scanning of the specimen using the electronbeam, a single scanning signal generating circuit may be set up togenerate an electron beam scanning signal (for shared use). The scanningsignal from the scanning signal generating circuit may be switchedalternately to the axial aligning process or to the specimen scanningprocess for use therein.

The scanning deflector 39 a is a single-stage deflector with a region ofelectron beam deflection of about 3 mm. The deflection needs to be suchthat the diameter of an differential evacuation aperture 50 will beexceeded. The exciting currents to the converging and objective lensesare set to zero in order to find that location (physical) on theobjective lens 4 which is illuminated by the electron beam from theelectron source 1. The electron beam deflected by the electron gundeflector 39 is shielded, if the amount of deflection turns out to behigh, by the differential evacuation aperture 50 at the bottom of thevessel 38. If the amount of deflection is small, the electron beampasses through the differential evacuation aperture 50 to reach theobjective lens 4.

Arriving at the objective lens 4, the electron beam hits an upper polepiece of the objective lens 4 to generate secondary electrons therefromprovided the range of electron beam deflection is greater than thediameter of that upper pole piece. If the range of electron beamdeflection is smaller than the diameter of the upper pole piece, thebeam passes therethrough without generating secondary electrons fromillumination. Thus when a secondary electron image is displayed bydetecting the intensity of secondary electrons in synchronism with thescanning of the electron gun deflector 39, the positions of the electronbeam and of the objective lens relative to the optical axis emerge asbright and dark concentric circles. The two-stage deflectors withreversal polarity 39 b are then used to align the vertical position andthe tilt angle of the electron beam relative to the optical axis.

The deflectors with reversal polarity 39 b make up a two-stage uprightarrangement. During alignment, the electron beam is vertically movedrelative to the optical axis by resorting to two-stage deflection withreversal polarity and by supplying the same exciting current to each ofthe two deflectors. The tilt angle of the electron beam relative to theoptical axis is adjusted by establishing an upper-to-lower deflectionamount such that a point (lower pivot position) at which the electronbeam deflected by the lower coil of the deflectors 39 b intersects theoptical axis will become the exact center of the differential evacuationaperture 50. When the dark-and-bright pattern of the secondary electronimage observed through electron beam scanning by the electron gundeflector 39 is adjusted to coincide with the center of the displayedimage, the electron beam from the electron source 1 is aligned with theoptical axis of the objective lens 4.

In the manner described, the electron gun deflector is used once forelectron beam alignment with the optical axis, and values representingthe conditions in effect after the alignment are stored into a storagedevice (not shown) as instructed by the CPU 14 through the interface 15.Upon observation of a specimen, the condition values are retrieved fromthe storage device so that operating conditions of the microscopecomponents are set up accordingly. This allows the microscope to startup and go into an observable state quickly. The swiftness with which themachine gets ready for use provides excellent usability as opposed toconventional TEMs. Because A/D and D/A converters are employed, theconditions determined and stored are reproduced more dependably thanbefore. This significantly reduces the frequency with which axialalignment needs to be adjusted.

The interface 15 and the CPU 14 may be furnished generally by a personalcomputer (PC). Where the scanning transmission electron microscope isdesigned to have its components controlled by use of a PC, the axialalignment of an electron beam upon on-site assembly of the machine maybe accomplished by qualified service personnel issuing instructions froma PC screen while turning a knob of a rotary encoder as needed. Whereonly qualified service personnel are allowed to perform axial alignment,an axial alignment icon on the PC screen may be arranged to demand entryof a password (e.g., personal ID) when manipulated. Such arrangementsrelieve the user of the chores of taking on axial alignment.

Described below is a layout of those detectors in the scanningtransmission electron microscope which detect electrons scattered orreflected by the specimen 5 upon electron beam illumination thereto, orsecondary electrons released anew from the specimen 5 following theillumination. FIG. 6 illustrates an electromagnetic lens circuit, lenscoils and detectors in conjunction with a single-stage projection lens8. The lenses are stacked in the vertical direction. A spacer 25 isinterposed between a converging lens 3 and an objective lens 4. Theconverging lens 3 is made up of a condenser lens magnetic circuit 19 anda condenser lens coil 20, while the objective lens 4 is composed of anobjective lens upper pole piece 21, an objective lens lower pole piece22, an objective lens magnetic circuit 23 and an objective lens coil 24.The spacer 25 accommodates a secondary electron detector 16 and areflected electron detector 17. The spacer 25 is provided not only tosecure space for the detectors installed, but also to exert shieldingeffects against an external alternating magnetic field because thespacer is made of permalloy. The spacer 25 also allows the deflectorcoil 12 to be installed.

The secondary electron detector 16 is oriented toward the specimen 5along the upper surface tilt of the objective lens upper pole piece 21.Thus positioned, the detector 16 detects secondary electrons extractedabove the objective lens upper pole piece 21 by a magnetic flux of theobjective lens 4. Inside the secondary electron detector 16 is anelectrode supplied with a high voltage of about 10 kV. The electrode iselectrically grounded using an insulator or the like, the groundingbeing independent of the electrical ground of the microscope.

An electric field formed by the electrode in the secondary electrondetector 16 extracts secondary electrons from the specimen and movesthem closer to the detector body to improve detection efficiency. Thereflected electron detector 17 is located in parallel with the top ofthe objective lens magnetic circuit 23. Because a detecting tip of thereflected electron detector 17 is located on the optical axis, the tiphas a through hole about 1 mm across centering on the optical axis sothat the hole allows the electron beam to pass through. Under theobjective lens 4 is the projection lens 8 made up of a projection lensmagnetic circuit 26 and a projection lens coil 27. Under the projectionlens 8 is an inner space of the vacuum vessel 28 accommodating thescattered electron detector 9 located on the optical axis. As with thereflected electron detector 17, the scattered electron detector 9 has adetecting tip that has a through hole about 3 mm across centering on theoptical axis so as to permit the transmitted electrons to pass through,whereby only the scattered electrons are detected.

FIG. 7 is a schematic view indicating approximate dimensions of theobjective lens 4. The coil (of about 1,800 turns) 24 measuringapproximately 180 mmφ in outer diameter, 80 mm in inner diameter and 75mm in elevation is housed inside a pure-iron magnetic circuit 23 forrotation having a diameter of 230 mmφ. In the innermost portion of themagnetic circuit 23 are the pole pieces 21 and 22 about 60 mm in outerdiameter each. The pole pieces 21 and 22 are formed by ahigh-permeability magnetic material called parmender. Because the shapesof the pole pieces 21 and 22 determine the performance of the objectivelens 4, deciding on dimensions “a” “b” and θ in FIG. 7 requires carefulconsideration. Depending on the design performance of the lens, thedimensions are set approximately to 1.5 to 16 mmφ across for thediameter “a” 3 to 10 mm for the portion “b” and 10 to 20 degrees for theangle θ. Where an electron beam with an acceleration voltage of 200 kVis used, the coil 24 is made to carry a current of about 7A to excitethe objective lens 4. In that case, a lens magnetic field with amagnetic flux density of about 1.5 T is formed between the pole piecesof the objective lens 4. A liquid coolant cooled to 12 to 13° C.circulates through the objective lens 4 to prevent excess temperaturerise.

Described below are ways to change the range of detection angle forscattered electrons in order to enhance a contrast of a scanningtransmitted image of a target portion to be observed. FIG. 8 is agraphic representation illustrating differences in angular distributionof scattering intensity between different kinds of materials, A and B.The differences concern all properties of each material: crystalstructures, kinds of component atoms, chemical composition, etc. Thematerial A is shown to have higher scattering intensities at mostscattering angles than the material B. This is a phenomenon oftenobserved when the material A has a greater atomic number than thematerial B. Inside a region X in FIG. 8, however, the material B has ahigher scattering intensity than the material A. That characteristic maybe utilized to observe a specimen wherein the materials A and B aremixed. That is, establishing the range of detection angle for scatteredelectrons in the region X allows information about the material B to beextracted with a high contrast.

What follows is a description of how to determine kinds of componentatoms within and a chemical composition of a specimen based on anacquired scanning transmission image intensity. A scanning transmittedimage of the specimen is first obtained using the above-describedscanning transmission electron microscope (STEM). At this point, therange of detection angle for scattered electrons is determined asfollows: exciting currents to the first and the second stage projectionlens 8 a and 8 b as well as a diameter of the objective aperture 11 areset so that the target portion to be observed will have a high contrast.The CPU 14 retrieves stored values representing image acquisitionconditions. Based on the retrieved values, the CPU 14 calculates valuesof exciting currents destined for the first and the second stageprojection lens 8 a and 8 b to obtain the focal length of each lens.When the magnification of an electron diffraction pattern (i.e., aminimum range for detecting scattered electrons) is calculated, thecalculated value is recorded along with the image obtained.

The acquired scanning transmitted image is analyzed as follows: inputfirst is information as to which stage of the objective aperture 11 hasbeen used (i.e., diameter of the aperture). Input next are elements ofsupposition that may be included in the acquired image as well as theircrystal structures. With these conditions entered, the CPU 14 computes ascattering intensity using as a parameter the stored range of detectionangle for scattered electrons, the range having been stored inconjunction with the scattering transmitted image.

The computing by the CPU is followed by designation, in the image file,of a specimen portion wherein types of component elements and theircrystal structures are known; these component elements and crystalstructures are input. If there are no known kinds of elements or crystalstructures, then an image file is prepared using known elements andcrystal structures observed under the same conditions as those for thetarget specimen; the contents of the prepared image file are used asreference data against which the scattering intensity of the inputelements under the same observing conditions is corrected.

A target portion to be analyzed is designated in the image file. Thetarget portion may be selected in terms of points, lines or areas. Ifdata denoting the component elements and crystal structures about thedesignated target portion coincide with any of the previously input dataon component elements and crystal structures, a message appears pointingout the data match. If there exist no matching data, another message isdisplayed to prompt the entry of data about the next candidate element.The operation is repeated until the composition of elements and thecrystal structures of the target portion to be analyzed have becomeknown.

Described below are ways to calculate and correct scattering intensitiesused in the above-described analysis. Where the types of componentelements and crystal structures constituting the specimen are known, itis possible to calculate the scattering intensity of interest based onthe so-called kinematic scattering theory or dynamic scattering theory.The kinematic scattering theory applies only to those materials havingan amorphous structure. The dynamic scattering theory, on the otherhand, is applicable to calculations of amorphous structures having along range order as well as to crystal structures. The calculating timeaccording to the kinematic scattering theory is extremely short comparedwith the computing time based on the dynamic scattering theory. For thatreason, the calculating and correcting methods based on the kinematicscattering theory should be selected when approximate solutions areneeded quickly; the methods as per the dynamic scattering theory need tobe selected if more accurate solutions are required.

The kinematic scattering theory presupposes that electrons are scatteredonly once by a specimen and that the probable angular distribution ofscattered electrons is proportional to the square of an atomicscattering amplitude of the scattered electrons. On these assumptions,the scattering intensity is determined solely by the kinds of elementsmaking up the specimen.

The calculations based on the dynamic scattering theory are somewhatcomplicated. A technique called multislice is used for the calculations.The multislice technique presupposes that a specimen is constituted by astack of thin slices about 0.5 nm or less in thickness each. Diffractionand propagation of electrons through each of the slices are calculatedrepeatedly. When all slices constituting the specimen have completed thecalculations, a wave function of the electrons is subjected to Fouriertransformation. The result is squared to provide a scattering intensity.Given an unknown material, one of the two scattering theories above isused to calculate the scattering intensity which in turn is used as abasis for analyzing the composition and crystal structure of thematerial of interest. It should be noted that because the scatteringintensity thus calculated is an intensity relative to an incidentelectron beam, the intensity of the incident electron beam must becorrected first.

The intensity of the incident electron beam is corrected with referenceto the above-mentioned image file containing values regarding knownelements and crystal structures. For example, if the measured intensityvalue is 100 in the image file while the calculated value is 10 percentof the incident electron beam (e.g., 0.1), then the coefficient forcorrection is 1,000. This correction coefficient is applied commonly toall input elements of the target to be analyzed.

Where the kinds of component atoms or crystal structures of the targetto be observed are already known, the input of such component atoms orcrystal structures allows the angular distribution of scatteredelectrons to be calculated. The result of the calculation is displayedon the PC screen. A desired angular position is then selected fromwithin the displayed angular distribution of scattered electrons. Thisdetermines a condition of excitation for the projection lens withrespect to the detection angle condition of interest, whereby a currentvalue for the projection lens is established. In this manner, thecontrast of the target portion to be observed may be enhanced or reducedas desired.

An application of the analysis of image contrast will now be described.For advanced devices, one of the factors that determine whether a givendevice is faulty or acceptable in characteristics is the densitydistribution of a dopant injected into the substrate. Illustratively,phosphorus (P) is used in silicon devices. The density distribution ofphosphorus is analyzed as a contrast of a scanning transmitted image byuse of differences in electron scattering power between silicon (Si) andphosphorus (P).

Silicon used in the substrate is single crystal in composition. Anelectron beam entering the substrate in the direction of its crystalorientation undergoes a strong Bragg reflection at low scatteringangles. This brings about a scattering intensity distribution with highscattering intensities within a range of low scattering angles. If thescattered electrons that fall within the range of low scattering anglesare selectively guided to the detector, a scanning transmitted imagewith a highlighted silicon contrast is obtained. Thus the scatteredelectrons within the range of low scattering angles are excluded fromthe target of detection when a distribution of phosphorus density in thesilicon substrate is to be observed. To exclude the scattered electronsat low scattering angles involves reducing the exciting current to theprojection lens and shortening the camera length (i.e., to reduce themagnification of the electron diffraction pattern) so as to prevent theelectrons in question from being detected by the detector. The procedureabove allows only high angle scattering components to be detected. Inthe resulting scanning transmitted image, contrasts of componentelements are known to be proportional to their atomic numbers.

That is, the contrast of silicon appears brighter than that ofphosphorus because the atomic number of silicon is the greater. Sincethe overall contrast of the image is proportional to the averagecomposition of phosphorus and silicon, the contrast varies linearly fromthe high to the low density portion of phosphorus. Thus the density ofphosphorus in a silicon substrate is analyzed by comparing twocontrasts: the actually measured contrast of the substrate; and acontrast between an image of scattering power distribution in thesilicon substrate measured under the same imaging conditions on the onehand, and an image of scattering power distribution in a specimen from asilicon substrate doped with phosphorus at a known density on the otherhand. A linear interpolated line is drawn between the images forcomparison and analysis.

Described below is how to observe internal structures of specimensthree-dimensionally using secondary or reflected electrons. An electronbeam entering a bulk specimen spreads in a teardrop fashion representinga region of reflected electron emission, as shown in FIG. 9. The higherthe acceleration voltage for the primary electron beam (probe), thedeeper the region of reflection electron emission. Illustratively, anacceleration voltage of 200 keV for a primary electron beam results in aregion of arrival several micrometers deep. The energy of secondaryelectrons stemming from the emission of the primary electron beam rangesfrom about 50 eV to hundreds of eV; secondary electrons occur solelywithin a region approximately up to 10 nm from a surface spot penetratedby the electron beam. These secondary electrons are detected and formedinto a scanning image by conventional low accelerating voltage scanningelectron microscopes. The scanning image constitutes a secondaryelectron image.

Simultaneously with the generation of secondary electrons, part of theprimary electron beam is reflected inside the specimen. The energy ofthe reflected electrons remains unchanged at 200 keV if the energy lossinside the specimen is ignored. The electrons intruding into thespecimen are reflected from inside and emitted as reflected electrons(or as secondary electrons formed near the specimen surface). Thereflected or secondary electrons are detected by a secondary chargedparticle detector whereby a high-accelerating voltage scanning electronmicroscope image is obtained.

Where the high-accelerating voltage scanning electron microscope isused, the yield of secondary electrons reflects not only informationabout the specimen surface but also information about an internalstructure of the specimen, as described. It is thus possible to extractthree-dimensional information about the interior of a specimen using ahigh-accelerating voltage scanning electron microscope to observe asecondary or a reflected electron image of the specimen.

The reason for reflected or secondary electron image thus observed toappear visually in a three-dimensional manner will now be described.Consider a case in which, as shown in FIG. 10, a foreign matter isincluded in a specimen and a primary electron beam (probe) is emitted topoints A, B and C in the specimen.

Point A appears bright because the foreign matter at this point islocated closest to the specimen surface and leaves secondary chargedparticles with a strong signal intensity. Point C appears dark becausethe foreign matter at this point is deep inside the specimen, allowingsecondary charged particles to have only a weak signal intensity.Changes in contrast occur in the depth direction of the specimen. When ascanning image is made of an internal structure of the specimen forobservation, the image is seen apparently with a depth. The deeper theobserved position from the specimen surface, the lower the spatialresolution of the matter, so that an image with an apparentlythree-dimensional perspective viewed from above is observed.

How three-dimensional observation is carried out will now be describedwith reference to the flowchart of FIG. 11. Initially, a specimenthinned by mechanical polishing and ion thinning (or by emission offocused ion beam) is set in a specimen-observing vessel of the scanningtransmission electron microscope to observe a scanning transmittedimage. If a scanning transmitted image is difficult to observe, thatmeans the specimen needs to be thinned further to let more electronspass therethrough. In that case, the specimen is additionally thinned.

The image observation and the additional thinning of the specimen arecarried out repeatedly, until an acceptable scanning transmitted imagecan be observed. The observation of the scanning transmitted image isfollowed by that of a secondary and a reflected electron image of thespecimen. Thereafter, the specimen is inverted in its position foranother observation of a secondary and a reflected electron image. Bythis time, a projected image of the overall specimen structure isacquired from the scanning transmitted image above, and so isinformation about a three-dimensional structure of the specimen down tocertain depths as viewed from its face as well as from its reverse sidebased on the secondary and reflected electron images. If it is judgedthat the three-dimensional structure as observed in the secondary andreflected electron images is not deep enough relative to the overallspecimen structure, then another observation of a secondary and areflected electron image is carried out by raising the accelerationvoltage of the primary electron beam. With the acceleration voltage atits highest setting, it may turn out that the depth of the structureobserved in secondary and reflected images is not yet sufficient. Inthat case, the specimen is thinned additionally.

As described, the invention envisages thinning specimens using a focusedion beam. This feature is implemented in the form of a specimen holderstructure for shared use by ion beam thinning equipment for specimenthinning and by the inventive STEM (scanning transmission electronmicroscope). A typical specimen holder structure and a matching specimenstage structure are described below.

FIG. 15 schematically shows a typical setup of a specimen holder and aspecimen stage designed for shared use by a scanning transmissionelectron microscope and by specimen thinning equipment utilizing FIB(focused ion beam). The specimen stage comprises a specimen drive forX-direction 45, a specimen drive for Y-direction 46, a specimen drivefor Z-direction 47 (the three drives constituting a triple-directiondrive system), and a specimen holder tilt system 48. These componentsare controlled by a stage controller 49. For observation of a scanningtransmission electron microscope image, a primary electron beam isemitted onto a specimen 5 in the negative Z-direction. For FIB machiningof the specimen 5, a focused ion beam (FIB) is emitted in the positiveY-direction.

The direction of additional thinning on the specimen starting from itsface or from its reverse side is determined by viewing secondary andreflected electron images acquired from the two sides. If an acquiredthree-dimensional image is judged to be deep enough based on theobserved secondary and reflected electron images, then the specimenholder tilt system 48 is activated to change the tilt angle of thespecimen 5. The specimen tilt angle is varied in order to acquiresecondary and reflected electron images observed in differentdirections. A plurality of images thus acquired in diverse directionsare put together for observation purposes.

The procedure above is effective where some aspects of the specimenstructure are already known or where observation of scanning transmittedimages of the specimen is needed. If the specimen structure is unknownor if the observation of a scanning transmitted image is not mandatory,then the specimen is observed three-dimensionally in accordance with theflowchart of FIG. 12. In this case, the acceleration voltage of theprimary electron beam is set for a maximum from the start, and asecondary and a reflected electron image of the specimen are observedfrom both sides thereof. A check is made of the observed images to seeif the target depth of the specimen has been properly observedthree-dimensionally. If the result of the check is satisfactory, thespecimen holder tilt system 48 is activated to change the direction ofobservation for another three-dimensional image observation as in thepreceding procedure.

If the target depth of the specimen has yet to be reached by observationwith three-dimensional images, the specimen is thinned further. Afterthe additional thinning, the specimen is again observed in secondary andreflected electron images. At this point, it is judged whether anadequate three-dimensional image of the target depth has been obtained,i.e., whether additional thinning of the specimen is needed. The processis repeated until the target depth of the specimen is properly observedthree-dimensionally. If further observation of the specimen withscanning transmitted images is deemed necessary, the observing processis continued.

An application of a three-dimensional observation will now be described.Semiconductor devices, for example, are often observed for such targetlocations as failed bits. When a specific location of a specimen is tobe observed in a scanning transmitted image, that location is thinnedusing a focused ion beam. During the specimen thinning process, anoverall specimen structure is checked and a currently machined depth ofthe specimen is verified repeatedly by observation with secondary andreflected electron images in accordance with the flowchart of FIG. 11.

The overall structure check and the depth verification repeated asoutlined above make it possible ultimately to fabricate a specimencontaining only such structures as gates and capacitors. Thesestructures may then be observed three-dimensionally.

Described above with reference to FIG. 12 were observations withsecondary and reflected electron images obtained by scanning a specimenusing a high accelerating voltage electron beam. Such observationspermit visually scrutinizing an internal structure of a thick specimenon the micrometer order; specimens of that kind are too thick to beobserved by a conventional transmission electron microscope.Observations based on the high accelerating voltage electron beam thusmake it possible to have three-dimensional verification of overallstructures in thick targets such as contacts in devices.

Below is a description of a typical layout of a secondary electrondetector based on a secondary electron detecting technique for improvingthe yield of secondary electrons. Secondary electrons are extracted fromthe specimen and moved above the objective lens by a magnetic flux ofthe pre-field objective lens that plays the part of demagnifying theprimary electron beam. In this setup, the secondary electron detector islocated above the objective lens as mentioned earlier. Although thesecondary electron detector located over the objective lens improves theyield of secondary electrons, the detector location is limited so as toprovide passage for the primary electron beam. Positioned close to thesecondary electron detector is an electrode supplied with a voltage forextracting secondary electrons. The higher the extracting voltage fed tothe electrode, the greater the yield of secondary electrons. However, anexcessively high extracting voltage can have adverse effects on thetrajectory of the primary electron beam. Where to locate the secondaryelectron detector and how much voltage to feed to its electrode arefactors that need careful consideration.

One way to study the location of the secondary electron detector withits electrode positioned nearby is by simulating a trajectory ofsecondary electrons in a setup that covers both the magnetic field ofthe objective lens and the electric field of the extracting electrodeclose to the detector. With the position of the specimen regarded as anelectron source, the energy of the secondary electrons therefrom issuitably postulated and their trajectory is calculated accordingly. Themagnetic flux density of the objective lens on the optical axis is knownto have a major effect on the trajectory of secondary electrons. Thus ifa magnetic flux distribution of secondary electrons is computedbeforehand using the finite element method, the trajectory of thesecondary electrons above the objective lens is obtained. After that,the effect of the electric field of the extracting electrode on thetrajectory of the primary electron beam is calculated. The results ofthe calculations are used to determine where to locate the detector withrespect to secondary electrons having a given level of energy. Similarcalculations are made for secondary electrons having different levels ofenergy so as to find hypothetical energy distributions of the secondaryelectrons. The processes above help determine an optimized position ofthe secondary electron detector together with its optimized dimensions.

Furthermore, the above calculations may be performed using as aparameter the voltage fed to the electrode for extracting secondaryelectrons. This additional computing makes it possible to determine inmore detail the optimized position of the secondary electron detectorand its optimized dimensions in keeping with the supplied voltage.

Another way to improve the yield of secondary charged particles is byutilizing a deflector having an electric field E and a magnetic field B(usually called an ExB filter or a Wien filter). This deflector deflectssecondary electrons from a specimen in the direction of a secondaryelectron detector without adversely affecting the trajectory of theprimary electron beam. The removal of the adverse effects is madepossible when the electric field E and the magnetic field B are arrangedto cancel out their influence with regard to the primary electron beam.

Described below is a technique for setting an optimized defocus amountin forming a fine probe on the basis of scanning transmitted imageintensities. FIG. 13 shows a specimen holding structure used for thistechnique. A mesh 35 for holding a specimen is a donut-shaped metalplate which is illustratively made of molybdenum (Mo) and which measuresapproximately 3 mmφ in outer diameter, 0.5 mmφ in inner diameter and 0.2mm in thickness. A crystal specimen 35 thinned for observation with ascanning transmitted image is bonded by adhesive or the like on top ofthe donut-shaped mesh 35 in such a manner that about half the centeropening is covered. Although not limited in terms of material, thecrystal specimen 34 should preferably have its crystal axis oriented inparallel with the incident direction of an electron beam 30. The latticedistance of the specimen 34 should be known in advance. In operation,the electron beam 30 is arranged to enter an edge of the crystalspecimen 34 for the observation of a scanning transmitted image at aninterface between the specimen and a vacuum space.

Preparatory to observation, the specimen unit shown in FIG. 13 is set inthe scanning transmission electron microscope. An edge of the crystalspecimen 34 close to the vacuum is viewed in a scanning transmittedimage at a 3,000,000x magnification for high-resolution observation.This reveals a bright-and-dark contrast of a scanning transmitted imageportion inside the specimen in keeping with the crystal latticedistance. Outside the specimen edge (i.e., in the vacuum space), asshown in FIG. 14, scattered electrons are also observed to peak inintensity.

The position in which the scattered electrons peak in intensity in thevacuum space is dependent on the defocus amount of the probe (i.e.,primary electron beam). The peak position shifts perpendicularly to avacuum interface (i.e., specimen edge) in accordance with the defocusamount. If focusing is correct with no diffraction aberration orspherical aberration (with no defocus), there should be no intensitysignal of scattered electrons observed outside the specimen edge. Inpractice, various aberrations lead to the observation of an intensitysignal of scattered electrons outside the specimen edge as illustratedin FIG. 14.

It follows that the state of focusing may be judged based on where ascattered electron intensity signal appears outside the specimen edge.That is, an under-focus is recognized if an intensity signal ofscattered electrons appears inside a given point F (i.e., on thespecimen side); an over-focus is detected if the scattered electronintensity signal is found outside the point F (on the vacuum side).

The scattered electron intensity signal appearing outside the specimenedge is a consequence of the sub-band position of the probe (primaryelectron beam) occurring in the scanning transmitted image. The sub-bandposition reflecting a defocus amount may be measured quantitatively inreference to the lattice difference of the crystal specimen, and adivergence between the sub-band position and an optimized defocus amountfor forming a fine probe may be calculated. When the exciting current tothe objective lens is modified so as to correct the divergence thuscalculated, an observation with the optimized defocus amount is madepossible. With the optimized defocus amount kept unchanged, the currentspecimen may be replaced by another specimen whose height may beadjusted for optimal focusing. This permits observation of a desiredspecimen with an optimized defocus amount.

The optimized defocus amount mentioned above is further explained below.The resolution of a scanning transmission electron microscope isdetermined by its probe diameter. The probe diameter varies with defocusamount. The optimized defocus amount refers to the amount of defocus atwhich the resolution is the highest, i.e., the probe diameter is thesmallest. The optimized defocus amount is zero where an ideal objectivelens with zero spherical aberration coefficient is used. In practice,objective lenses have their inevitable spherical aberrations. In thecircumstances, defocusing is carried out to balance diffractionaberration with spherical aberration, whereby the probe diameter isminimized. Since spherical aberration is specific to each microscope,each machine has its own optimized defocus amount.

If Cs stands for the spherical aberration coefficient of an objectivelens and λ for the wavelength of an electron beam, then an optimizeddefocus amount Δf is given by the following equation: $\begin{matrix}{{\Delta \quad f} = {- \sqrt{{Cs} \cdot \lambda}}} & (1)\end{matrix}$

The intensity profile of the probe χ(u) is obtained by subjecting afunction defined by equation (2) below to Fourier transformation and bysquaring the transformed result, where “u” denotes a spatial frequency.The profile thus acquired permits calculating the sub-band position ofthe probe having an optimized defocus amount.

χ(u)=Exp[−i·πλ{(1/2) Cs·λ ² u ² +Δf}] . . . . . (2)

Comparing the result of the calculation above with an actual measurementprovides a divergence from the optimized defocus amount. The divergencevalue thus obtained is fed back to the microscope for correction.

As described above in detail, the scanning transmission electronmicroscope according to the invention dispenses with thosetime-consuming chores of axial alignment which required experience andexpertise. Users are relieved of the tedious task of alignment, and thespeed of measurement and observation with the microscope is boosted.

According to the invention, the range of detection angle for scatteredelectrons is selected by changing the exciting current to the projectionlens. This improves the contrast of the target portion to be observed ina scanning transmitted image.

From the features described above, the following practical benefits arederived:

(1) A resolution of 0.24 nm is made available on a monitor screen.

(2) Observations are made and images are recorded with the microscopeinstalled in a well-lighted room. A dark room is no longer necessary.

(3) An insulating film of semiconductor devices (e.g., gate oxide film)is observed with a high contrast using a conventional TEM method.

(4) An ultra-thin film on the nanometer order is directly measured withthe microscope.

In addition, because the inventive scanning transmission electronmicroscope is controlled by a personal computer (PC), a PC screen isused to issue instructions for image transmission, filing, commententry, measurement and printing through digitally displayed images. Thescreen-driven order input provides the machine with excellent usability.

Upon acquisition of an image, the imaging parameters determined by theexciting current to the projection lens are automatically analyzed bythe CPU and recorded in conjunction with the acquired image. The storedparameters are retrieved and used for subsequent structural andcomponent analyses. This feature promises highly accurate analyses.

Secondary and reflected electron images may be obtained using anelectron beam of a high accelerating voltage. This makes it possible forthe internal structure of a specimen to be observed as an image having athree-dimensional perspective on the micrometer order.

A secondary electron image, a reflected electron image and a scanningtransmitted image of a specimen may be observed in combination using aprimary electron beam of a high accelerating voltage. This feature helpsdetermine an appropriate amount and direction of corrective machining ona specimen.

While preferred embodiments of the invention have been described usingspecific terms, such description is for illustrative purposes only, andit is to be understood that changes and variations may be made withoutdeparting from the spirit or scope of the appended claims.

What is claimed is:
 1. A scanning transmission electron microscopyapparatus, comprising: an electron source means for generatingelectrons; an electron extracting means for extracting an acceleratedprimary electron beam from said electron source means; an electronilluminating system for illuminating said accelerated primary electronbeam extracted from said electron source means onto a specimen to beobserved; a specimen holder for holding said specimen; a first electrondeflecting system for scanning said specimen with said acceleratedprimary electron beam illuminated onto said specimen; a scatteredelectron detector for detecting scattered electrons transmitted throughsaid specimen; a secondary electron detector for detecting secondaryelectrons released from said specimen; a reflected electron detector fordetecting reflected electrons from said specimen; and a second electrondeflecting system for aligning said accelerated primary electron beam tothe optical axis of the scanning transmission electron microscopyapparatus; and wherein said electron extracting means includes anelectron accelerating electrode assembly for accelerating said primaryelectron bean, said electron illuminating system includes an electronconverging lens system for converging said accelerated primary electronbeam and an objective lens system for demagnifying and illuminating saidaccelerated and converged primary electron beam onto said specimen, andsaid second electron deflecting system is disposed between said electronaccelerating electrode assembly and said electron converging lenssystem, and wherein said second electron deflecting system used forscanning said accelerated primary electron beam on the upper surface ofsaid objective lens system in order to align said accelerated primaryelectron beam to said optical axis of said scanning transmissionelectron microscopy apparatus.
 2. A scanning transmission electronmicroscopy apparatus according to claim 1, wherein said objective lenshas an upper pole piece disposed over said specimen and a lower polepiece disposed under said specimen.
 3. A scanning transmission electronmicroscopy apparatus according to claim 1, wherein said scatteredelectron detector is a detector using a CCD or a harpicon camera.
 4. Ascanning transmission electron microscopy apparatus, comprising: anelectron source means for generating electrons; an electron extractingmeans for extracting an accelerated primary electron beam from saidelectron source means, said electron extracting means includes anelectron accelerating electrode assembly for accelerating said primaryelectron beam; an electron illuminating system for illuminating saidaccelerated primary electron beam extracted from said electron sourcemeans onto a specimen to be observed, the electron illuminating systemincludes an electron converging lens system for converging saidaccelerated primary electron beam and an objective lens system fordemagnifying and illuminating said accelerated and converged primaryelectron beam onto said specimen; a specimen holder for holding saidspecimen; a first electron deflecting system for scanning said specimenwith said accelerated primary electron beam illuminated onto saidspecimen; a secondary electron detector for detecting secondaryelectrons released from said specimen; a second electron deflectingsystem for scanning said accelerated primary electron beam on the uppersurface of said objective lens, the second electron deflecting system isdisposed between said electron accelerating electrode assembly and saidelectron converging lens system; an alignment means for aligning saidaccelerated primary electron beam to said optical axis of the scanningtransmission electron microscopy apparatus in response to the detectionsignal of said secondary electron detector which is obtained when saidaccelerated primary electron beam is being scanned by said secondelectron deflecting system.
 5. A scanning transmission electronmicroscopy apparatus according to claim 4, further comprising: animaging means for forming a scanning secondary electron image based onthe detection signal of said secondary electron detector which isobtained when said accelerated primary electron beam is being scanned bysaid second electron deflecting system.
 6. A scanning transmissionelectron microscopy apparatus according to claim 4, wherein saidacceleration voltage of said accelerated primary electron beam is set toat least 200 kV.
 7. A scanning transmission electron microscopyapparatus according to claim 4, wherein said objective lens has an upperpole piece disposed over said specimen and a lower pole piece disposedunder said specimen.
 8. A scanning transmission electron microscopyapparatus, comprising: an electron source means for generatingelectrons; an electron extracting means for extracting an acceleratedprimary electron beam from said electron source means, said electronextracting means including an electron accelerating electrode assemblyfor accelerating said primary electron beam; an electron illuminatingsystem for illuminating said accelerated primary electron beam extractedfrom said electron source means onto a specimen to be observed, saidelectron illuminating system including an electron converging lenssystem for converging said accelerated primary electron beam and anobjective lens system for demagnifying and illuminating said acceleratedand converged primary electron beam onto said specimen; a specimenholder for holding said specimen; a first electron deflecting system forscanning said specimen with said accelerated primary electron beamilluminated onto said specimen; a scattered electron detector fordetecting scattered electrons transmitted through said specimen; asecondary electron detector for detecting secondary electrons releasedfrom said specimen; a reflected electron detector for detectingreflected electrons form said specimen; and a second electron deflectingsystem for aligning said accelerated primary electron beam to theoptical axis of the scanning transmission electron microscopy apparatus,said second deflecting being disposed between said electron acceleratingelectrode assembly and said electron converging lens system, whereinsaid second electron deflecting system is provided with a scanningdeflector and two-stage deflectors with reversal polarity located aboveand below the scanning deflector, for scanning said accelerated primaryelectron beam on the upper surface of said objective lens system inorder to align said accelerated primary electron beam to the opticalaxis of said scanning transmission electron microscopy apparatus.
 9. Ascanning transmission electron microscopy apparatus comprising: anelectron source means for generating electrons; an electron extractingmeans for extracting an accelerated primary electron beam from saidelectron source means, said electron extracting means including anelectron accelerating electrode assembly for accelerating said primaryelectron beam; an electron illuminating system for illuminating saidaccelerated primary electron beam extracted for said electron sourcemeans onto a specimen to be observed, said electron illuminating systemincluding an electron converging lens system for converging saidaccelerated primary electron beam and an objective lens system fordemagnifying and illuminating said accelerated and converged primaryelectron beam onto said specimen; a specimen holder for holding saidspecimen; a first electron deflecting system for scanning said specimenwith said accelerated primary electron beam illuminated onto saidspecimen; a secondary electron detector for detecting secondaryelectrons released from said specimen; a second electron deflectingsystem provided with a scanning deflector and two-stage deflectors withreversal polarity located above and below the scanning deflector, forscanning said primary accelerated primary electron beam on the uppersurface of said objective lens system, said second deflecting beingdisposed between said electron accelerating electrode assembly and saidelectron converging lens system; and an alignment means for aligningsaid accelerated primary electron beam to the optical axis of thescanning transmission electron microscopy apparatus in response to thedetection signal of said secondary electron detector which is obtainedwhen said accelerated primary electron beam is being scanned by saidsecond electron deflecting system.